CSE 331 Software Design & Implementation

CSE 331Software Design & Implementation Hal Perkins Autumn 2013 Generics (Polymorphism) (Slides by Mike Ernst and David Notkin)

Varieties of abstraction Abstraction over computation: procedures int x1, y1, x2, y2; Math.sqrt(x1*x1 + y1*y1); Math.sqrt(x2*x2 + y2*y2); Abstraction over data: ADTs (classes, interfaces) Point p1, p2; Abstraction over types: polymorphism (generics) Point<Integer>, Point<Double> Applies to both computation and data

Why we ♥ abstraction Hide details Avoid distraction Permit the details to change later Give a meaningful name to a concept Permit reuse in new contexts Avoid duplication: error-prone, confusing Programmers hate to repeat themselves – “lazy”

Programs include a group of abstractions Declares a new variable, called a formal parameter interface ListOfNumbers{ boolean add(Number elt); Number get(int index); } interface ListOfIntegers{ boolean add(Integer elt); Integer get(int index); } … and many, many more interface List<E> { boolean add(E n); E get(int index); } Instantiate by passing an Integer: lst.add(7); aList.add(anInt); The type of add isInteger  boolean Declares a new type variable, called a type parameter Instantiate by passing a type: List<Float> List<List<String>> List<T> The type of List isType  Type

Type variables are types Declaration class NewSet<T> implements Set<T> { // rep invariant: // non-null, contains no duplicates List<T> theRep; T lastItemInserted; } Use Use

Declaring and instantiating generics // a parameterized (generic) class public class Name<TypeVar,..., TypeVar> { Convention: 1-letter name such as:TforType,Efor Element,N for Number,K for Key,V for Value,MforMurder Class code refers to type parameter by name, e.g., E To instantiate, client supplies type arguments e.g., String as in Name<String> Analogous to “constructing” a specific class from the generic definition

Restricting instantiations by clients boolean add1(Objectelt); boolean add2(Numberelt); add1(new Date()); // OK add2(new Date()); // compile-time error interface List1<E extends Object> {…} interface List2<E extends Number> {…} List1<Date> // OK, Date is a subtype // of Object List2<Date> // compile-time error, // Date is not a subtype // of Number Upper bounds

Using type variables Code can perform any operation permitted by the bound interface List1<E extends Object> { void m(E arg) { arg.asInt(); // compiler error, E might not // support asInt } } interface List2<E extends Number> { void m(E arg) { arg.asInt(); // OK, since Number and its // subtypes support asInt } }

Another example public class Graph<N> implements Iterable<N> { private final Map<N, Set<N>> node2neighbors; public Graph(Set<N> nodes, Set<Tuple<N,N>> edges) { ... } } public interface Path<N, P extends Path<N,P>> extends Iterable<N>, Comparable<Path<?, ?>> { public Iterator<N> iterator(); } Do NOTcut/paste this into your project unless it is what you want (and you understand it!)

Bounded type parameters <Type extends SuperType> An upper bound; accepts the given supertype or any of its subtypes Works for multiple superclass/interfaces with & <Type extends ClassA & InterfaceB & InterfaceC & ...> <Type super SuperType> A lower bound; accepts the given supertype or any of its supertypes Example // tree set works for any comparable type public class TreeSet<T extends Comparable<T>> { ... }

Not all generics are for collections Class Utils { static NumbersumList(List<Number> lst) { Numberresult = 0; for (Numbern : lst) { result += n; } return result; } }

Signature of a generic method Class Utils { static T sumList(Collection<T> lst) { // ... black magic within ... } } Type uses Where is this type variable declared? Where is this type variable declared?

Declaring a method’s type parameter class MyUtils { static <T extends Number> T sumList(Collection<T> lst){ // ... black magic within ... } } How to declare a type parameter of a method

Sorting public static <Textends Comparable<T>> void sort(List<T> list) { // … use list.get() and T.compareTo(T) } Actually: <T extends Comparable<? super T>>

Generic methods public static <Type> returnType name(params) { When you want to make just a single (often static) method generic in a class, precede its return type by type parameter(s) public class Collections { ... public static <T> void copy(List<T> dst, List<T> src) { for (T t : src) { dst.add(t); } } }

More bounded type examples <T extends Comparable<T>>T max(Collection<T> c) Find max value in any collection (if the elements can be compared) <T> void copy( List<T2 super T> dst, List<T3 extends T> src) Copy all elements from src to dst dst must be able to safely store anything that could be in src This means that all elements of src must be of dst's element type or a subtype <T extends Comparable<T2 super T>>void sort(List<T> list) Sort any list whose elements can be compared to the same type or a broader type

Generics and subtyping Integer is a subtype of Number Is List<Integer> a subtype of List<Number>? Use subtyping rules (stronger, weaker) to find out List<Number> Number ? List<Integer> Integer

List<Number> and List<Integer> interface List<Number> { boolean add(Number elt); Number get(int index); } interface List<Integer> { boolean add(Integer elt); Integer get(int index); } Java subtyping is invariant with respect to generics i.e, not covariant, not contravariant

Invariant subtypingis restrictive Solution: wildcards Unrelated to invariant subtyping interface Set<E> { //Adds all of the elements in c to this set // if they're not already present. void addAll(Set<E> c); void addAll(Collection<E> c); void addAll(Collection<? extends E> c); <T> void addAll(Collection<T extends E> c); } A wildcard is essentially an anonymous type variable Use it when you would use a type variable exactly once It appears at the use site; nothing appears at the declaration site Problem 1: Set<Number> s; List<Number> l; s.addAll(l); Caused by invariant subtyping Problem 2: Set<Number> s; List<Integer> l; s.addAll(l);

Using wildcards class HashSet<E> implements Set<E> { void addAll(Collection<? extends E> c) { // What can this code assume about c? // What operations can this code invoke on c? ... } } A wildcard is essentially an anonymous type variable Wildcards are written at type argument uses Within a parameter declaration A missing extends clause means “extends Object” There is also “? super E”

Legal operations on wildcard types Object o; Number n; Integer i; PositiveInteger p; List<? extends Integer> lei; First, which of these is legal? lei = new ArrayList<Object>; lei = new ArrayList<Number>; lei = new ArrayList<Integer>; lei = new ArrayList<PositiveInteger>; lei = new ArrayList<NegativeInteger>; Which of these is legal? lei.add(o); lei.add(n); lei.add(i); lei.add(p); lei.add(null); o = lei.get(0); n = lei.get(0); i = lei.get(0); p = lei.get(0);

Legal operations on wildcard types Object o; Number n; Integer i; PositiveInteger p; List<? super Integer> lsi; First, which of these is legal? lsi= new ArrayList<Object>; lsi= new ArrayList<Number>; lsi= new ArrayList<Integer>; lsi= new ArrayList<PositiveInteger>; lsi= new ArrayList<NegativeInteger>; Which of these is legal? lsi.add(o); lsi.add(n); lsi.add(i); lsi.add(p); lsi.add(null); o = lsi.get(0); n = lsi.get(0); i = lsi.get(0); p = lsi.get(0);

Wildcards ? indicates a wild-card type parameter, one that can be any type List<?> list = new List<?>(); // anything Difference between List<?> and List<Object> ? can become any particular type; Object is just one such type List<Object> is restrictive; wouldn't take a List<String> Difference between List<Foo>and List<? extends Foo> The latter binds to a particular Foo subtype and allows ONLY that Ex: List<? extends Animal> might store only Giraffes but not Zebras The former allows anything that is a subtype of Foo in the same list Ex: List<Animal> could store both Giraffes and Zebras

PECS: Producer Extends, Consumer Super Where should you insert wildcards?Should you use extends or super or neither? • Use ? extends T when you getvalues from a producer • Use ? super T when you putvalues into a consumer • Use neither (just T, not ?)if you both getand put Example: <T> void copy(List<? super T> dst, List<? extends T> src)

Equals for a parameterized class class Node { … @Override public booleanequals(Object obj) { if (!(objinstanceofNode)) { return false; } Node n = (Node) obj; return this.data().equals(n.data()); } … }

Equals for a parameterized class Erasure: at run time, the JVM has no knowledge of type arguments class Node<E> { … @Override public booleanequals(Object obj) { if (!(objinstanceofNode<E>)) { return false; } Node<E> n = (Node<E>) obj; return this.data().equals(n.data()); } … }

Equals for a parameterized class class Node<E> { … @Override public booleanequals(Object obj) { if (!(objinstanceofNode<?>)) { return false; } Node<E> n = (Node<E>) obj; return this.data().equals(n.data()); } … } Erasure again. At run time, equivalent to Node<Elephant> type = (Node<String>) obj;

Equals for a parameterized class class Node<E> { … @Override public booleanequals(Object obj) { if (!(objinstanceofNode<?>)) { return false; } Node<?> n = (Node<?>) obj; return this.data().equals(n.data()); } … } Works if the type of obj is Node<Elephant> or Node<String> or … Node<? extends Object> Node<Elephant> Node<String> no subtyping relationship

Arrays and subtyping Number Integer Integer is a subtype of Number Is Integer[] a subtype of Number[]? Use our subtyping rules to find out (Same question as with Lists) Same answer with respect to true subtyping Different answer in Java! Integer[]is a Java subtype of Number[] Java subtyping disagrees with true subtyping Number[] ? Integer[]

Integer[] is a Java subtype of Number[] Number n; Number[] na; Integer i; Integer[] ia; na[0] = n; na[1] = i; n = na[0]; i = na[1]; ia[0] = n; ia[1] = i; n = ia[0]; i = ia[1]; ia = na; Double d = 3.14; na = ia; na[2] = d; i = ia[2]; Why did the Java designers do this?

Tips when writing a generic class Start by writing a concrete instantiation Get it correct (testing, reasoning, etc.) Consider writing a second concrete version Generalize it by adding type parameters Think about which types are the same & different Not all ints are the same, nor are all Strings The compiler will help you find errors Eventually, it will be easier to write the code generically from the start but maybe probably not yet

Parametric polymorphism “Parametric polymorphism” means: identical code and behavior, regardless of the type of the input Applies to procedures and types One copy of the code, many instantiations Utilizes dynamic dispatch Types of parametric polymorphism Dynamic (e.g., Lisp) static (e.g., ML, Haskell, Java, C#, Delphi) C++ templates are similar; both more and less expressive In Java, called “generics” Most commonly used in Java with collections Also used in reflection and elsewhere Lets you write flexible, general, type-safecode

Generics clarify your code interface Map { Object put(Object key, Object value); equals(Object other); } interface Map<Key,Value> { Value put(Keykey, Valuevalue); equals(Object other); } Generics usually clarify the implementation sometimes ugly: wildcards, arrays, instantiation Generics always make the client code prettier and safer plus casts in client code → possibility of run-time errors Cost: More complicated declarations and instantiations, added compile-time checking

Generics Java practicalities

Type erasure All generic types become type Object once compiled Big reason: backward compatibility with old ancient byte code So, at runtime, all generic instantiations have the same type List<String> lst1 = new ArrayList<String>(); List<Integer> lst2 = new ArrayList<Integer>(); lst1.getClass() == lst2.getClass() // true You cannot use instanceof to discover a type parameter Collection<?> cs = new ArrayList<String>(); if (csinstanceof Collection<String>) { // illegal }

Generics and casting Casting to generic type results in a warning List<?> lg = new ArrayList<String>(); // ok List<String> ls = (List<String>) lg; // warn The compiler gives an unchecked warning, since this isn't something the runtime system is going to check for you Usually, if you think you need to do this, you're wrong (Unless you’re implementing things like ArrayList– and then be sure you understand why you’re getting the warning) The same is true of type variables: public static <T> T badCast(T t, Object o) { return (T) o; // unchecked warning }

Generics and arrays public class Foo<T> { private T aField; // ok private T[] anArray; // ok public Foo(T param) { aField = new T(); // error anArray = new T[10]; // error } } You cannot create objects or arrays of a parameterized type (Actual type info not available at runtime)

Generics/arrays: a hack public class Foo<T> { private T aField; // ok private T[] anArray; // ok @SuppressWarnings("unchecked") public Foo(T param) { aField = param; // ok T[] anArray = (T[])(new Object[10]); // ok } } You can create variables of that type, accept them as parameters, return them, or create arrays by casting Object[] Casting to generic types is not type-safe, so it generates a warning You almost surely don’t need this in common situations!

Comparing generic objects public class ArrayList<E> { ... public intindexOf(E value) { for (inti = 0; i < size; i++) { // if (elementData[i] == value) { if (elementData[i].equals(value)) { return i; } } return -1; } } When testing objects of type E for equality, must use equals

CSE 331 Software Design & Implementation